1. Field of the Invention
The invention relates to an image forming apparatus such as an electrophotographic printer having means for certainly preventing print dirt even if the apparatus is in a stationary state for a predetermined period.
2. Related Background Art
Generally, in an image forming apparatus such as an electrophotographic printer or the like, a photosensitive drum comes into contact with a charging roller and is, then, charged, an electrostatic latent image is written onto the photosensitive drum by an exposing unit, a toner image is formed onto the electrostatic latent image by a developing apparatus comprising a developing roller, a developing blade, and the like, and the toner image is transferred onto a print medium by a transfer apparatus comprising a transfer roller and a transfer belt, thereby printing.
The toner remaining on the photosensitive drum after the transfer is collected by pushing a cleaning roller or a cleaning blade of a cleaning apparatus onto the rotating photosensitive drum (for example, refer to JP-A-07-56491).
Most of the image forming apparatuses such as an electrophotographic printer and the like use a contact type developing system in which a developing roller constructed by forming a rubber elastic layer onto a conductive shaft comes into contact with a photosensitive drum by a predetermined pressing force and toner is developed.
However, in the conventional image forming apparatuses, there is such a problem that in a portion where the developing roller comes into contact with the photosensitive drum (hereinbelow, such a portion is referred to as a nip portion), since they are always in the contact state, if they are left for a long time without executing printing, low molecular components (hereinafter, referred to as “oligomer”) precipitated from a rubber material of the developing roller are deposited onto the photosensitive drum, so that no dot can be formed due to defective exposure in a halftone image in a 1-by-1 mode or a 2-by-2 mode in the first printing. There is also such a problem that since a lateral stripe is formed, quality of the print image deteriorates (hereinafter, such a state is referred to as an “oligomer line”).